The documents EPO694 792 A1 and the corresponding U.S. Pat. No. 5,736,695 A reveal such a device using ultrasonic energy. A sound signal generator at the elevator car couples sound pulses into a sound conductor, for example in the form of a metal wire. A receiver at the upper or lower end of the elevator path receives the sound pulses. On the basis of the known speed of sound in the sound conductor and the sound traveling or propagation time of the pulses measured, it is possible to calculate the distance between signal generator and receiver and thus the position of the elevator car in the elevator path.
To be able to measure the sound propagation time, the measuring means must be capable of unequivocally associating a sound pulse received at the end of the elevator path with a specific transmitted pulse. In case of this known device, this is ensured in that the time interval between two pulses transmitted is greater than the sound propagation time from one end to the other end of the sound conductor. Thus, there can always be only one sound pulse on the sound conductor at a time, and this sound pulse has to belong to the pulse transmitted last.
It is disadvantageous in this respect that a lower measurement value actualization rate results with longer elevator traveling distances. This renders the measurement slow and sensitive to occasional interference and white noise, for example, quantization errors in the signal processing operation.
That the transmission of sound pulses at spaced time intervals greater than the sound propagation time between the two ends of the sound conductor causes problems in elevators with long travel path, can be seen, for example, from the elevator installed in the Munich Olympic Tower, which has a travel path of approx. 200 meters and moves at a speed of 7 m/s. Assuming a sound propagation time of 20 ms per 100 m length of a metal wire, a sound propagation time of 40 ms between lower end and upper end of the travel path results for the approx. 200 m long travel path of the elevator of the Olympic Tower. With a time interval between the sound pulses coupled successively into the metal conductor which is greater than the sound propagation time between both ends thereof, consecutive sound pulses would have to have a time interval of more than 40 ms. With a running speed of 7 m/s, the elevator car would move on 28 cm between the transmission of two consecutive sound pulses. For modern elevator systems in which the elevator car is to be controlled with an accuracy of 1 mm, a detection of the elevator car position every 28 cm along the travel path only, is completely insufficient.
It is known from DE 199 03 645 A1 and the corresponding CA 2296472 A1 to transmit measurement pulses having the same time interval from each other that is shorter than the sound propagation time in the sound conductor from one end to the other end of the elevator path in order to obtain a higher actualization rate. This has the result that there is always a plurality of sound pulses on the metal wire serving as sound conductor at the same time. In order to be able to assign each of these measurement pulses to a specific transmitted measurement pulse on the receiving side, synchronization pulses are transmitted in addition to these measurement pulses, with the distances in time between the same being greater than the maximum propagation time of a sound pulse from one end to the other end of the sound conductor and with these synchronization pulses being different from the measurement pulses by a predetermined feature. For example, each synchronization pulse has a time interval from the measurement pulses adjacent the same, which is different from the time interval between adjacent measurement pulses. For example, the respective synchronization pulse is in the middle of the time interval between two adjacent measurement pulses. Thus, a measurement pulse received can be associated unequivocally with the synchronization pulse transmitted last. The measurement pulses between two consecutive synchronization pulses may then be associated on the receiver side with a specific measurement pulse transmitted by way of their identification number in relation to the respective synchronization pulse.
This method is not without disadvantages, either. On the one hand, an association of a receiving pulse with a specific transmission pulse is possible only after arrival of the corresponding synchronization pulse. On the other hand, this method is sensitive to disturbances due to reflected pulses, especially with regard to the fact that the received pulses usually do not have ideal pulse edges, but impaired pulse edges. Reflections are caused, for example, in that the sound conductor indeed is terminated at both ends thereof by attenuation members, but these do not completely absorb the sound pulses, but reflect the same in part. Such reflections have the result that pulses not belonging to the same transmission pulse meet at specific locations along the sound conductor. If disturbing interference arises between transmission pulses and reflection pulses at a specific location along the sound conductor, this interference holds for all measurement pulses, due to the same time interval between the measurement pulses.
The hardware and software requirements for the evaluation algorithm are determined by the smallest time interval between two adjacent pulses. The shorter this interval, the higher the processing clock rate needs to be and the higher the requirements for hardware and software and thus for the costs for the same. Due to the fact that, in the known method, the respective synchronization pulse is placed between the two measurement pulses adjacent the same, hardware and software have to be designed for a processing rate corresponding to the brief distances or intervals in time between a synchronization pulse and the measurement pulses adjacent the same. Hardware and software thus need to be of more complex design than required for processing of the measurement pulses alone. I.e., for processing the measurement pulses proper, it would be sufficient to have hardware and software that could be much less complex if there were no synchronization pulses.